With increasing interest in alternative energy sources and for many years applications, it is desirable to store hydrogen in a metal by subjecting the metal or alloy thereof, to a temperature and pressure and which contact with hydrogen will cause hydrogen to be absorbed by the metallic composition. The latter can then be stored and subjected to another set of temperature and pressure conditions at which hydrogen will be released in a gaseous state, i.e. made available for whatever purpose the hydrogen can serve, e.g. as a fuel or for some industrial, chemical or metallurgical application.
It is known, for example, that a titanium-iron alloy of substantially the composition TiFe (equiatomic) is able to absorb hydrogen at ambient temperature and at a pressure of 20 to 50 bar (see U.S. Pat. No. 3,516,263).
The storage capacity of the TiFe alloy can vary significantly between, say, 1 and 1.75% of hydrogen (by weight of the alloy) depending upon the purity of the alloy and the purity of the hydrogen.
A TiFe alloy of this type can be used for more than 3000 absorption/desorption cycles without reduction in its storage capacity when the system is in a sealed vessel, although the capacity is found to drop when the hydrogen in contact with the alloy is changed from cycle to cycle.
It is thus assumed that impurities present in the hydrogen tend to inhibit absorption at ambient temperature in the Ti-Fe alloy and results in the formation at more elevated temperatures of oxides such as Ti.sub.10 Fe.sub.7 O.sub.3, whose presence reduces the absorption capacity and hence the ability of the alloy to store hydrogen.
At page 31 of Hydrogen, Its Technology and Implications, Vol. II, "Transmission and Storage," published by CCR Press, Cleveland, Ohio, it is indicated that the presence of amounts as small as 0.01% (100 ppm) of oxygen in gaseous hydrogen is able to significantly inhibit the metal-hydrogen reaction of the TiFe system.
Storage systems using TiFe alloys, therefore, have practically been limited to the use of anhydrous hydrogen with the purity of at least 99.99%.
For instance, instructions for the use of an AHT-5 type reservoir with titanium-iron hydride, developed by the Billings Research Corporation, Utah, exclude the possibility of using this system for technical grade hydrogen with a purity generally of about 99.5% and which can contain 0.5% air.
In U.S. Pat. Nos. 3,922,872 and 4,079,523, proposals have been made for modifying the TiFe alloy including various iron substituents, especially manganese and/or nickel. Various studies have been made of ternary alloys based on TiFe and containing as the tertiary metal a transition metal substituent for Fe. These afforts are described by
G. D. Sandrock: "Metallurgical Considerations in the Production and Use of FeTi Alloys for Hydrogen Storage," published in Proc. 11th International Energy Conversion and Engineering Conference, pp. 965-971, by American Institute of Chemical Engineers, New York, 1974;
J. J. Reilly: "Titanium Alloy Hydrides and Their Applications," published in Proc. First World Hydrogen Energy Conference, by Verizroglu T. N. Ed. University of Miami, Coral Gables, Flor., 1976.
The state of the art represented by these publications illustrates that it is possible to reduce the equilibrium pressure of the hydride further by the TiFe alloy, when chromium, manganese, cobalt, nickel or copper are substituted in part for the iron. Thus, ternary alloys can be used for the storage of hydrogen at reduced pressure from that which would be required for the TiFe alloy alone by the introduction of one or another of these substituents mentioned above.
However, experiments with such ternary systems have demonstrated that problems are encountered with the storage of hydrogen in several senses.
Firstly, oxidizing impurities adversely affect the storage capacity for large quantities of hydrogen. Secondly, the capacity for the storage of hydrogen of such ternary alloys cannot satisfactorily be maintained for hydrogen of technical grade over a large number of absorbtion/desorption cycles.
Thirdly, the alloys are not always satisfactory for the storage of hydrogen at desirable temperature and pressure levels and at a reasonable cost.